1. Field of the Invention
The present invention relates to longitudinally coupled resonator-type surface acoustic wave filters, and more particularly, relates to a longitudinally coupled resonator-type surface acoustic wave filter having three or more interdigital transducers (xe2x80x9cIDTsxe2x80x9d).
2. Description of the Related Art
Conventionally, surface acoustic wave filters have been widely used as an RF stage band-pass filter of a mobile phone. Low insertion loss, large attenuation, wideband, and other such characteristics are required for the band-pass filter. To satisfy these requirements, various propositions have been made for the surface acoustic wave filter.
In Japanese Unexamined Patent Application Publication No. 5-267990, one example of a method for achieving a wider band of a longitudinally coupled resonator-type surface acoustic wave filter is disclosed. In this publication, electrode fingers are arranged to periodically line up between IDTs that are adjacent to each other, and a process for optimally providing a resonant mode by causing the center distance between the adjacent electrode fingers of two IDTs that are adjacent to each other in the propagation direction of a surface acoustic wave to be deviated by about 0.5 times of the wavelength determined by the period of the electrode fingers is disclosed.
However, when, as described above, the center distances between adjacent electrode fingers of IDTS that are adjacent to each other is deviated by about 0.5 times of the wavelength determined by the period of the electrode fingers, the periodical continuity of the surface acoustic wave propagation path is worsened in the corresponding part. Particularly, when a piezoelectric substrate, such as a 36xc2x0 Y-cut X-propagating LiTaO3 or a 64xc2x0 Y-cut X-propagating LiNbO3, making use of a leaky surface acoustic wave (leaky wave) is used, loss due to bulk wave radiation increases. As a result, although realization of a wider band can be intended, there is a problem in that insertion loss increases.
In order to overcome the problems described above, preferred embodiments of the present invention provide a longitudinally coupled resonator-type surface acoustic wave filter which solves the foregoing problems, so that not only the realization of the wider band but also significant reduction in the insertion loss in a passband can be achieved.
In a longitudinally coupled resonator-type surface acoustic wave filter according to a preferred embodiment of the present invention, since at least one IDT among at least three IDTs is constructed so that the electrode finger period of a first portion that is adjacent to the side edge of another IDT in the propagation direction of the surface acoustic wave is different from the electrode finger period of a second portion that is the remaining portion of the IDT, it is possible to not only achieve the expansion of the passband width but also greatly reduce the insertion loss in the passband.
Therefore, the longitudinally coupled resonator-type surface acoustic wave filter having a wider band and low loss in the passband is achieved.
When the electrode finger period of the first portion is shorter than the electrode finger period of the second portion, the propagation loss of the surface wave is greatly reduced. This enables the insertion loss in the passband to be even more reduced.
Particularly, when the electrode finger period of the first portion is approximately 0.82 to about 0.99 times the electrode finger period of the second portion, the propagation loss of the surface wave can be even more reduced.
When the center distance between adjacent electrode fingers of a pair of adjacent IDTs is caused to be substantially equal to 0.5xcexI1, the loss radiated as a bulk wave is greatly reduced, which reduces the insertion loss even more.
When only one of a pair of the IDTs which are adjacent to each other is constructed so as to include the first portion and the second portion and when the center distance between adjacent electrode fingers of the pair of adjacent IDTs is caused to be substantially equal to 0.25xcexI1+0.25xcexI2, the loss radiated as the bulk wave is greatly reduced in the same manner, which reduces the insertion loss in the passband by an even larger amount.
When the electrode finger distance is substantially equal to 0.25 xcexI1+0.25 xcexI2 in a portion in which electrode fingers of the first portion and electrode fingers of the second portion are adjacent to each other, the loss radiated as the bulk wave can be reduced in the same manner, which more reduces the insertion loss in the passband.
When the polarity of the electrode fingers adjacent to each other of the IDT including the first portion and the second portion and the polarity of the electrode fingers of the IDT adjacent to the IDT are different, the surface acoustic wave is converted into an electric signal in the spacing between adjacent IDTs as well. This increases the conversion efficiency into the electric signal and improves the insertion loss in the passband even more. In addition, the passband width is expanded.
When the total number of electrode fingers of the first portions is not more than 18 on both sides of the adjacent portions of a pair of the IDTs which are adjacent to each other, the concentration ratio of the impedance is increased whereby the longitudinally coupled resonator-type surface acoustic wave filter having small VSWR can be provided.
When the center distance between the electrode fingers, having periods that are not different, of a pair of the IDTs which are adjacent to each other is (0.08+0.5n)xcexI2 to (0.24+0.5n)xcexI2, more preferably, (0.13+0.5n)xcexI2 to (0.23+0.5n)xcexI2, a necessary bandwidth can be obtained in accordance with various applications such as an EGSM method, a DCS method, and a PCS method, and VSWR can be positively reduced.
When both of a pair of the IDTs, which are adjacent to each other in the propagation direction of the surface acoustic wave filter, include the first portion and the second portion, and the numbers of electrode fingers of the first portions of both IDTs are different, though the concentration ratio of the impedance as well as VSWR are worsened, the expansion of the passband width can be achieved even more readily.
When what is obtained by rotating a LiTaO3 single crystal in the direction of the Y axis in the range of approximately 36xc2x0 to 44xc2x0 with respect to the X axis is used as the piezoelectric substrate, the longitudinally coupled resonator-type surface acoustic wave filter having a wide passband width and low insertion loss in the passband can be easily obtained according to various preferred embodiments of the present invention.
When the film thickness of the electrode fingers of the first portion is different from the film thickness of the electrode fingers of the second portion, the loss due to the radiation of the bulk wave is greatly reduced by adjusting the electrode film thickness. Particularly, when the film thickness of the electrode fingers of the first portion is caused to be thinner than the film thickness of the electrode fingers of the second portion, since the loss due to the radiation of the bulk wave can be reduced in the spacing between IDTs in which the bulk wave is most likely to be generated, the insertion loss can be even more reduced while the wide bandwidth is maintained.
When, in a construction in which multiple-stage longitudinally coupled resonator-type surface acoustic wave filters are cascaded, the electrode finger period of the first portion of at least one stage of the longitudinally coupled resonator-type surface acoustic wave filter is caused to be different from the electrode finger period of the first portion of another stage thereof, the passband width can be expanded even more without causing the deterioration in VSWR.
When the electrode finger period of the first portion is different in each stage of the multiple-stage longitudinally coupled resonator-type surface acoustic wave filter, the passband width can be effectively expanded.
When at least one series resonator and/or parallel resonator is connected to an input side and/or output side, not only the reduction in the insertion loss in the passband but also the expansion of the out-of-passband attenuation amount can be achieved according to various preferred embodiments of the present invention.
The longitudinally coupled resonator-type surface acoustic wave filter according to various preferred embodiments of the present invention may be constructed so as to have a balanced-unbalanced input/output or a balanced-balanced input/output.
That is, various input/output types of longitudinally coupled resonator-type surface acoustic wave filters can be easily provided in accordance with various applications.
Since a communication apparatus according to a preferred embodiment of the present invention is provided with the longitudinally coupled resonator-type surface acoustic wave filter constructed according to various preferred embodiments of the present invention as a band-pass filter, the communication apparatus having a wider bandwidth and the low loss can be constructed.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
Other elements, characteristics, features and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.